6.772/SMA5111 - Compound Semiconductors
Lecture 1 - The Compound Semiconductor Palette - Outline
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• Announcements
Handouts General Information; Syllabus; Lecture 1 Notes
• Why are semiconductors useful to us?
(Why isn't Si enough?)
Review of the properties of silicon
Quantifying the importance of silicon to the electronics industry
Representative applications silicon is not suitable for
(...at least not yet)
• Which materials are semiconductors?
(What are our choices?)
Elemental semiconductors
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Compound semiconductors - binaries
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1. III-V's; 2. II-IV's; 3. IV-VI's; 4. I-VII's
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Alloy semiconductors
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1. Ternaries; 2. Quarternaries; 3. Others: a) More than 4; b) Si-Ge
• Properties vs. composition
(Making sense of all the options)
Crystal structure
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Energy band structure
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Carrier type and transport
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Optical properties
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Other
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C. G. Fonstad, 2/03 Lecture 1 - Slide 1

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• Physical, structural
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Crystal structure
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Lattice period (Å)
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• Energy levels
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Energy gap (eV)
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Band symmetry
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Density of states (cm -3 )
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• Electrical, charge carriers
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Low field mobility (cm 2 /V-s)
Critical E-field (V/cm)
Saturation velocity (cm/s)
Effective mass (relative)
• Optical
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Absorption edge ( l gap
)
Radiative lifetime (s)
Typical radiative Efficiency (%) diamond
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5.431
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1.1
� indirect gap
N = 2.8 x 10 19 N = 1.02 x 10 19 m l m t
Electrons
1450
10 4
10 7
�
0.98
�
0.19
� m lh m hh
Holes
450
5 x 10 4
10 7
0.16
0.5
1.1 µm few ms
<<1%
C. G. Fonstad, 2/03 Lecture 1 - Slide 2
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• Light emitters
Light emitting diodes, Laser diodes any wavelength
• Mid- and far-infrared detectors
( l
≥ 1.1 µm)
Fiber communication wavelengths
Atmospheric windows l = 1.3 and 1.55 µ m l
= 3 to 5 µm and 8 to 12 µm
Infrared imaging arrays night vision
Thermophotovoltaic cells responding to 500 K black bodies
• Ultraviolet detectors
( l
≤ 0.5 µm)
Solar blind detectors no response in visible
• Optical modulators
Amplitude modulation of light
• Very-high speed electronics
Systems operating at 40 GHz and above
• High temperature electronics
Operable at temperatures above 200˚C
• Cryogenic electronics
Operating at 4.2 K and below for fiber telecomm for fiber telecomm process monitoring space instrumentation
C. G. Fonstad, 2/03 Lecture 1 - Slide 3
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• Elemental semiconductors
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Column IV: C (diamond), Si, Ge, Sn (grey)
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All have the diamond structure:
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All are indirect band gap
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(Image deleted)
See Fig 3a in: Sze, S.M. Semiconductor Devices, Physics and Technology
New York, Wiley, 1985 .
(Image deleted)
See Fig 1-5-6 in: Shur, M.S. Physics of Semiconductor Devices
Englewood Clifs, N.J., Prentice-Hall, 1990 .
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Notice the trend
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Sn: ~0.08 eV
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Ge: 0.67 eV
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Si: 1.12 eV
� g
):
C: 5.5 eV
Diamond and Ge are useful, but we will say little about them.
C. G. Fonstad, 2/03 Lecture 1 - Slide 4
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• Binary compounds
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The choices are many-
Column III with column V (the three-fives, III-V's) : A
II
B
III
B
V
Column II with column VI (the two-sixes, II-VI's): A
Column IV with Column VI (the four-sixes, IV-VI's): A
Column I with Column VII: A
I
B
VII
VI
IV
B
(these are insulators)
VI
III IV V VI
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To help us make sense of all these options we will find that there are clear trends
(a method to the madness)
B
5
C
6
N
7
O
8
II
Al Si P S
13 14 15 16 The best way to start is by looking at plots of lattice period vs. energy gap...
Zn Ga Ge As Se
30 31 32 33 34
E g
Cd In Sn Sb Te
48 49 50 51 52
C. G. Fonstad, 2/03 a
Hg Tl Pb Bi Po
80 81 82 83 84
Lecture 1 - Slide 5
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Compound Semiconductors:
The zinc blende lattice
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(Image deleted)
See Fig 3a in: Sze, S.M. Semiconductor Devices, Physics and Technology
New York, Wiley, 1985 .
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Diamond lattice
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(Image deleted)
See Fig 3b in: Sze, S.M. Semiconductor Devices, Physics and Technology
New York, Wiley, 1985 .
Zinc blende lattice
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(GaAs shown)
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C. G. Fonstad, 2/03 Lecture 1 - Slide 6
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Compound Semiconductors:
Direct vs indirect bandgaps
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(Image deleted)
See Fig 1-5-6 in: Shur, M.S. Physics of Semiconductor Devices
Englewood Cliffs, N.J., Prentice-Hall, 1990 .
C. G. Fonstad, 2/03 Lecture 1 - Slide 7
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Binary Compound Semiconductors: Zinc-blende III-V's II-VI's
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ZnSe
AlP
2.5 CdS
ZnTe
0.5
AlAs
2.25
GaP
2.0
1.75
1.5
1.25
GaAs
InP
CdSe
AlSb
0.6
0.7
CdTe
0.8
0.9
1.0
Si
1.0
1.5
0.75
GaSb
0.5
Ge
2.0
3.0
InAs
0.25 InSb
10.0
0.54 0.56 0.58 0.60
Lattice period, a (nm)
0.62 0.64
C. G. Fonstad, 2/03 Lecture 1 - Slide 8
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Binary Compound Semiconductors: Zinc-blende III-V's II-VI's
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Material
System
Semiconductor
Name Symbol
Crystal Lattice
Structure Period(A)
Energy Band
Gap(eV) Type
III-V Aluminum phosphide AlP
Aluminum arsenide AlAs
Aluminum antimonide AlSb
Gallium phosphide GaP
Gallium arsenide GaAs
Gallium antimonide GaSb
Indium phosphide
Indium arsenide
InP
InAs
Indium antimonide InSb
Z
Z
Z
Z
Z
Z
Z
Z
Z
5.4510
5.6605
6.1355
5.4512
5.6533
6.0959
5.8686
6.0584
6.4794
2.43
2.17
1.58
2.26
1.35
0.36
0.17 i i i i
1.42 d
0.72 d d d d
II-VI Zinc sulfide
Zinc selenide
Zinc telluride
ZnS
ZnSe
ZnTe
Cadmium sulfide CdS
Cadmium selenide CdSe
Cadmium telluride CdTe
Z
Z
Z
Z
Z
Z
5.420
5.668
6.103
5.8320
6.050
6.482
3.68 d
2.71 d
2.26 d
2.42 d
1.70 d
1.56 d
Key: Z = zinc blende; i = indirect gap, d = direct gap
C. G. Fonstad, 2/03 Lecture 1 - Slide 9
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Additional Semiconductors:
Wurzite III-V's and II-VI's
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Lead Salts (IV-VI's), Column IV
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Material
System
Semiconductor
Name Symbol
Crystal Lattice
Structure Period(A)
Energy Band
Gap(eV) Type
III-V Aluminum Nitride AlN
(nitrides)
Gallium Nitride GaN
Indium Nitride InN
II-VI Zinc Sulfide ZnS
(wurtzite)
Cadmium Sulfide CdS
W
W a = 3.189, c = 5.185 3.36
W
W
W a = , c = a = , c =
6.2
0.7
a = 3.82, c = 6.28 3.68
a = 4.16, c = 6.756 2.42 i d d d d
IV-VI Lead Sulfide
IV
Lead Selenide
Lead Telluride
Diamond
Silicon
Germanium
Grey Tin
PbS
PbSe
PbTe
C
Si
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Ge
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Sn
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IV-IV Silicon Carbide SiC
Silicon-Germanium Si x
Ge
1-x
D
D
D
D
R
R
R
5.9362
6.128
6.4620
3.56683
5.43095
5.64613
6.48920
0.41
0.27
0.31
5.47
1.124
0.66
0.08 d d d
W a = 3.086, c = 15.117 2.996 i
Z vary with x (i.e. an alloy) i i i i d
Key: Z = zinc blende, W = wurtzite, R = rock salt; i = indirect gap, d = direct gap
C. G. Fonstad, 2/03 Lecture 1 - Slide 10
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Binary Compound Semiconductors: mobility trends
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(Image deleted)
See Fig 1 in: Sze, S.M. ed., High Speed Semiconductor Devices
New York, Wiley, 1990 .
(Image deleted)
See Fig 2 in: Sze, S.M. ed., High Speed Semiconductor Devices
New York, Wiley, 1990 .
C. G. Fonstad, 2/03 Lecture 1 - Slide 11
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• Binary compounds
Most have direct bandgaps.
(very important to optoelectronic device uses)
They cover a wide range of bandgaps, but only at discrete points.
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They follow definite trends
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They can be grown in bulk form and cut into wafers.
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We still need more….
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• Ternary alloys
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Not compounds themselves, but alloys of two binary compounds with one common element.
(ternary compounds are of limited interest)
Ternary alloys have two elements from one column, one from another and there are two options:
(III-V examples)
A
A
III(1-x)
B
III(x)
C
V
III
B
V(1-y)
C
V(y)
{= [A
III
{= [A
III
B
C
V
]
(1-x)
+ [B
V
]
(1-y)
+ [A
III
C
V
]
III
C
V
]
(x)
}
(y)
}
With ternary alloys we have access to a continuous range of bandgaps
C. G. Fonstad, 2/03 Lecture 1 - Slide 12
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Ternary Alloy Semiconductors:
3 III-V examples, AlGaAs, InGaAs, InAlAs
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Three III-V ternaries: InGaAs, AlGaAs, InAlAs
2.5
AlAs
2
AlGaAs
1.5
GaAs
InAlAs
InP
InGaAs
AlGaAs (direct
InAlAs(direct)
InAlAs (indirect)
AlGaAs (indirect)
Binaries
1
InGaAs
0.5
InAs
0
5.6 5.7 5.8 5.9
Lattice period (Angstoms)
6 6.1
C. G. Fonstad, 2/03 Lecture 1 - Slide 13

Ternary trends:
Lattice period: linear with composition (Vegard's Law)
Band gaps: quadratic with composition; slope and curvature vary with band minima
C. G. Fonstad, 2/03 Lecture 1 - Slide 14
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Ternary trends:
Most properties, such as effective mass, vary quadraticly and monotonically with alloy fraction.
Alloy scattering is largest near a 50% mix and transport properties tend to not vary monotonically.
Æ �
C. G. Fonstad, 2/03 Lecture 1 - Slide 15

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• Ternary alloys
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Give us access to continuous ranges of bandgaps,
� but as E g varies so in general does a.
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Substrates are always binary and only come at discrete a's.
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Thus to grow heterostructures we need different E g layers, all with the same a, and ternaries don't do the full job.
(Note: AlGaAs is an important exception; since it is intrinsically lattice-matched to
GaAs it was used in the first heterostructure work. However, soon more was needed...)
• Quarternary alloys
Quarternaries mix 4 elements - there are 2 types:
(III-V examples)
1. 2 elements from one column, 2 from the other:
(mixes of 4 binaries)
A
III(1-x)
B
III(x)
C
V(1-y)
D
V(y)
{= [A
III
C
V
+[B
III
C
]
(1-x)(1-y)
V
] x(1-y)
+[A
+[B
III
III
D
V
D
V
]
]
(1-x)y xy
}
2. 3 elements from one column, 1 from the other:
(3 binary mixes)
A
A
III(1-x-y)
III
B
B
III(x)
C
III(y)
D
V(1-x-y)
C
V(x)
D
V(y)
V
{= [A
III
D
V
]
(1-x-y)
+[B
III
{= [A
III
B
V
]
(1-x-y)
+[A
III
C
D
V
V
]
]
(x)
(x)
+[C
III
D
V
+[A
III
D
V
]
]
(y)
(y)
}
}
With quarternary alloys we have access to ranges of in materials that are all lattice-matched to a binary substrate
C. G. Fonstad, 2/03 Lecture 1 - Slide 16
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III-V quarternaries: InGaAlAs
C. G. Fonstad, 2/03 Lecture 1 - Slide 17
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III-V quarternaries: more examples InGaAsP and AlGaAsSb
C. G. Fonstad, 2/03 Lecture 1 - Slide 18

III-V quarternaries: more examples GaInAsSb
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C. G. Fonstad, 2/03 Lecture 1 - Slide 19
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III-V quarternaries: more examples GaAlAsP and GaAlInP
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C. G. Fonstad, 2/03 Lecture 1 - Slide 20
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The III-V wurtzite quarternary:
GaInAlN
AlN
0.2
6.0
5.0 0.25
0.3
4.0
GaN
3.0
2.0 InN
1.0
0.28
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C. G. Fonstad, 2/03
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0.30 0.32 0.34
Lattice period, a (nm)
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0.36 0.38
�
0.4
0.5
0.6
0.7
(Image deleted)
See Fig 2a: Sze, S.M. Physics of
Semiconductor Devices, 2nd ed.
New York: Wiley, 1981
Lecture 1 - Slide 21
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• All have uses, but some are more widely used than others
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GaAs-based heterostructures
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InP-based heterostructures
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Misc. II-IVs, III-Vs, and others
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• Important Binaries
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GaAs
InP substrates, MESFETs substrates
GaP red, green LEDs
• Important Ternaries and Quaternaries
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AlGaAs on GaAs HBTs, FETs, optoelectronic (OE) devices
GaAsP on GaAs red, amber LEDs
HgCdTe on CdTe
InGaAsP, InGaAlAs on InP
InGaAlAs on InP
InGaAs on GaAs, InP
IR imagers
OEs for fiber telecomm. ditto ohmic contacts, quantum wells
InGaAsP on GaAs red and IR lasers, detectors
GaInAlN on various substrts. green, blue, UV LEDs, lasers
C. G. Fonstad, 2/03 Lecture 1 - Slide 22
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6.772/SMA5111 - Compound Semiconductors
Lecture 1 - The Compound Semiconductor Palette - Summary
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• Why are semiconductors useful to us?
Unique electrical and optical properties we can control
Silicon falls short in light emission and at performance extremes
(...at least so far)
• Which materials are semiconductors?
Elemental semiconductors: Si, Ge, Sn
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Compound semiconductors - binaries
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1. III-V's; 2. II-IV's; 3. IV-VI's; 4. I-VII's
Alloy semiconductors: 1. Ternaries; 2. Quarternaries; 3. Si-Ge
• Properties vs. composition
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General observation - energy gap increases, lattice period decreases as move up periodic table and out from Column IV
Crystal structure - determines compatibility; heterostructure feasibility; lattice spacing varies linearly with alloy composition
Energy band structure - important for electrical and optical properties
Carrier type and transport - narrower gap implies higher electron mobility; hole mobilities change little; cannot always have p-type when gap is large
Optical properties - direct band-gaps essential for some applications;
indirect band-gaps appear in wider band gap materials
C. G. Fonstad, 2/03 Lecture 1 - Slide 23
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